Technical Abstract:
Fish are constantly under threat from a large number of pathogens consisting of bacteria, viruses, fungi, and parasites. However skin and mucus are barriers to pathogens attempting to invade the body from the environment. The innate immune responses in and on the skin are the next line of defense against invading pathogens. Macrophages, granulocytes, complement, C-reactive protein, lectins, lysozyme, antibiotic-like proteins, and iron-binding transferring are among the nonspecific components of this immunity. These components act quickly in killing and removing the pathogens. This defense has no recall response aginst previous invading pathogens. Immunostimulants may cause temporary enhancement of innate immune responses against pathogens. When innate immune defenses are penetrates, the next line of defense is adaptive immunity, a specific defense mechanism. Components of adaptive immunity system are macrophages, lymphocytes, cytokines, and antibodies. Adaptive defense has recall responses against previous invading pathogens. The adaptive defense response is initiated by macrophages in cooperation with lymphocytes. Subsequently, lymphocytes replicate and antibody is produced to eliminate the invading pathogen. Adaptive immunity may also be a cellular defense mediated by macrophages and lytic lymphocytes. Vaccination initiates both antibody and cellular adaptive defenses to eliminate pathogens. Vaccines composed of antigenic components of pathogens (killed vaccine) initiate antibody defenses, whereas vaccine composed pf modified 'live pathogens (avirulent vaccine) initiate antibody and cellular adaptive defenses. For example, Streptoccus iniae infection of tilapia is prevented by a killed S. iniae vaccine that initiates a protective antibody response. On the other hand, Edwardsiella ictaluri infection of catfish (ESC) is prevented by an avirulent E. ictaluri vaccine that initiates a protective cellular response. However, these vaccines and others may fail because the immune system of fish is vulnerable to dysfunction. Loss of immune system resilience and surveillance against pathogens results in disease. Among the major reasons for the dysfunction of the immune system is stress. Environmental and husbandry stressors weaken both the innate and adaptive immune responses against pathogens. Stress may be controlled by minimizing husbandry stressors such as high stocking densities, excessive handling, confinement, transportation, and drug administration, during periods of environmental stressors, including changes in water temperature, dissolved gases, pH, and salinity. Because full recovery from stress responses, such as those caused by hypoxia, may take days to weeks, the likelihood of infectious epidemics may be high if fish farmers resume stressful husbandry practices too quickly after prolonged and intense exposure to environmental stressors. Stress weakens the defenses of fish against pathogens; thus, stress management is essential in keeping the fish immune system in a healthy state.